Catalyst ash protector

ABSTRACT

A combustion apparatus includes: a combustion device operable to produce flue gas having large particulate ash entrained therein; an outlet duct coupled in flow communication with the combustion device so as to receive the flue gas; a screen positioned at least partially in the outlet duct downstream of the combustion device, the screen effective to trap large particulate ash while permitting flow of the flue gas therethrough; and apparatus operable to remove large particulate ash from the screen while at least a portion of the screen remains inside the duct.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Provisional Patent Application No. 61/241,071, filed Sep. 10, 2009.

BACKGROUND OF THE INVENTION

This invention relates generally to NOx emissions reduction in combustion processes, and more particularly to the improvement of performance of Selective Catalytic Reduction (SCR) systems.

Fossil fuel combustion processes, such as electric power generation, produce numerous types of atmospheric pollutants as by-products, in particular oxides of nitrogen (NOx). Environmental regulations are becoming increasingly more stringent, requiring power plants to reduce stack emissions even further. Therefore, it is desirable to improve and enhance the methods and equipment currently used for removing pollutants from gas streams.

One particular type of emissions-reduction equipment is a Selective Catalytic Reduction (SCR) system. SCR systems are the only NOx reduction technology capable of achieving greater than 90% reductions of NOx emissions. A major issue with SCR systems is the accumulation of large particle ash (LPA) deposits originating from the combustion process on the SCR system, which impedes the performance of the SCR system.

Current technology utilizes fixed screens to prevent the LPA deposits from contacting the SCR system. A drawback of this technology is the build up of LPA particles on the screens, which cause performance of the overall system to degrade over time as LPA accumulates on the screen.

BRIEF SUMMARY OF THE INVENTION

These and other shortcomings of the prior art are addressed by the present invention, which provides a catalyst protection system incorporating moving screens that can be cleaned continuously, to prevent build up of build up of LPA.

According to one aspect of the invention, a combustion apparatus includes: a combustion device operable to produce flue gas having large particulate ash entrained therein; an outlet duct coupled in flow communication with the combustion device so as to receive the flue gas; a screen positioned at least partially in the outlet duct downstream of the combustion device, the screen effective to trap large particulate ash while permitting flow of the flue gas therethrough; and apparatus operable to remove large particulate ash from the screen while at least a portion of the screen remains inside the duct.

According to another aspect of the invention, a method of removing large particulate ash from a gas stream includes: passing a combustion gas stream having large particulate ash entrained therein through a duct; collecting large particulate ash on a screen positioned at least partially in the duct, while permitting flow of the flue gas therethrough; and removing the ash from the screen at a location external to the duct while at least a portion of the screen remains inside the duct.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:

FIG. 1 is a schematic block diagram of a combustion process incorporating a catalyst ash protector constructed according to an aspect of the present invention; and

FIG. 2 is an enlarged view of a portion of an outlet duct and catalyst ash protector shown in FIG. 1; and

FIG. 3 is a view the outlet duct and catalyst ash protector of FIG. 1 in a vertical orientation.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1 depicts an exemplary combustion process incorporating a catalyst protection system constructed according to an aspect of the present invention. As shown, the combustion process 10 comprises a combustion device 12, such as a fossil-fuel-fired boiler, that uses air to combust fuel, such as a fossil fuel, for example, coal. The combustion device 12 produces a gas stream in the form of flue gas that exits the combustion device 12 through an outlet duct 14 (represented schematically by an arrow in FIG. 1). The flue gas produced within the combustion device 12 includes air; gaseous products of combustion, such as water vapor, carbon dioxide, oxides of nitrogen (NOx) and sulfur, halides, organic compounds, mercury, selenium, and other trace metal vapors; and aerosols, including particulate matter or fly ash and in some cases sulfuric acid mist; and large particulate ash (LPA). As used herein, the term “large particulate ash” refers to solid-phase combustion byproducts in the form of particles having a normal size of about 6 mm (0.23 in.) to about 10 mm (0.39 in.).

A selective catalyst reduction (SCR) unit 16 is positioned downstream of the combustion device 12. The SCR is of a known type which functions to reduce oxides of nitrogen (NOx) in the flue gas to nitrogen and oxygen. Downstream of the SCR unit 16 is an air preheater 18 which serves to recover heat from the flue gas by transferring the heat to the combustion air which is sent to the combustion device 12 along with the fuel supply. Finally, downstream of the air preheater 18 is a particulate collection device 20, such as a wet or dry electrostatic precipitator or a baghouse, which removes aerosols and particulate matter from the flue gas. The flue gas exiting the particulate collection device 20 exits to a stack 22 and thence to the atmosphere. It should be appreciated that this combustion process is only exemplary, and the concepts of the present invention may be used with combustion processes having various configurations of specific equipment.

As noted above, the SCR unit 16 is susceptible to blockage or clogging by large particulate ash (LPA). Accordingly, a catalyst ash protector 24 is positioned in the outlet duct 14 between the combustion device 12 and the SCR unit 16. It serves to protect downstream selective catalyst reduction unit (SCR) 18 from blockage by removing LPA.

FIGS. 2 and 3 illustrate the catalyst ash protector 16 in more detail. The catalyst ash protector 16 may be used with ducting in any orientation. In FIG. 2, the outlet duct 14 is shown in a horizontal orientation, and in FIG. 3, it is shown in a vertical orientation. The main component is a screen 26 which extends across the outlet duct 14. The screen 26 may be mounted in any orientation (e.g. vertical or horizontal), and need not be mounted perpendicular to the direction of flow through the outlet duct 14.

The screen 26 comprises a plurality of holes that allow the gas to pass through the screen 26. The size, shape, and position of the holes in the screen 26 can be varied to optimize the performance of the screen 26 and the gas pressure drop across the screen 26. In general, the holes should be less than about 10 mm (0.39 in.) in diameter so as to effectively trap LPA particles. Preferably, the holes have a diameter ranging from about 0.1 mm (0.004 in.) to about 10 mm (0.4 in.), more preferably about 1 mm (0.04 in.) to about 8 mm (0.3 in.). Alternatively, the screens may be a felted cloth or metal material with a high gas permeability, for example, approximately 100 cfm/ft² of screen or greater. It should be appreciated that the shape of the holes and the hole pattern may also be varied as desired. For example, the holes could be simple squares or other shapes such a diamond, rectangles, etc. The hole pattern or pitch in the screen may also vary. The materials of construction used for the screen 26 may also be varied and selected based upon the gas composition, such as the corrosivity of the gas, and to optimize the structural integrity of the screen 26. For example, the screen 26 may be constructed from metal wire mesh; woven cloth made from glass, ceramic, carbon, or polymer yarn; or holes punched in a metal or polymer sheet. Given the various options for constructing the screen 26, it should be appreciated that the term “screen” is used throughout generically and should not be construed as limited to, for example, a metal wire screen.

The screen 26 may be wrapped around two rollers 28 and 30 near the boundaries of the outlet duct 14. The rollers 28 and 30 may be mounted inside or outside the outlet duct 14. An electric motor 32 or other suitable drive mechanism is provided to drive the roller 28 (and thus the screen 26) such that, when operating, each portion of the screen moves along a continuous path from one roller 28, across the outlet duct 14 to the other roller 30, and back again to the first roller 28. A hopper 34 may be connected or positioned under a portion of the outlet duct 14 such that the screen 26 may move from the outlet duct 14 through the hopper 34 and back into the outlet duct 14. The hopper 34 may be filled with water or a wash solution to clean the screen 26 as it rotates through the hopper 34. When operated in the position shown in FIG. 3 the hopper 34 may include a slot or opening to allow the screen 26 to pass through its side wall. Alternatively, the hopper 34 could incorporate spray nozzles (not shown) to enhance the cleaning of the screen 26. The hopper 34 may be fed with water or a wash solution by a feed line (not shown) and dirty water or spent wash solution may be discharged from the hopper 34 through a discharge line (not shown).

In operation, the screen 26 may be rotated through the hopper 34 as desired. For example, such rotation may be done on a regularly scheduled basis, either manually or automatically. Alternatively, rotation may be done continuously or only on an as-needed basis depending upon, for example, the gas pressure drop across the screen 26. Other alternative apparatus and methods of cleaning the screen 26 include air jets, vacuum cleaning, mechanical brushing or scraping, vibration cleaning, or configuration of the screen and surrounding hardware so that LPA may fall off by gravity. Box 35 in FIG. 2 represents schematically LPA removal apparatus positioned to access the screen 26. This may be used in addition to or instead of the hopper 34. As another alternative the screen may be fed from a roll of screen material and move in only one direction onto another roller on the other side. The screen can still be cleaned on the rollers as described previously. Once the roll of screen is depleted, the direction of the screen is simply reversed. Alternatively the spent screen roll can also be disposed of or refurbished off site as needed.

In addition to plugging or blocking by LPA particles, the SCR system 16 is also subject to poisoning and loss of effectiveness from vapor phase contaminants in the flue gas stream such as arsenic, selenium, and nickel. To address this problem, alkaline material such as lime may be injected into the flue gas upstream of the catalyst. FIG. 1 shows a schematic view of an injector 36 positioned for this purpose. The alkaline material would be provided as a powder or particulate having a size similar to that of the LPA, for example greater than 0.1 mm (0.004 in.), preferably greater than 1.0 mm (0.04 in.). In the outlet duct 14, this material is believed to bind with the vapor phase contaminants, especially arsenic. The alkaline material may then be removed by the catalyst ash protector 24 in the same way the LPA is removed.

The moving screen catalyst ash protector described herein has several advantages as opposed to currently used stationary screens which are subject to plugging. The moving screen will avoid misdistribution of the flue gas, high pressure drops across the screen 26, and screen deterioration, which can only be rectified during a plant outage.

The foregoing has described a catalyst protection system for a combustion process and a method for its operation. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only. 

1. A combustion apparatus comprising: a combustion device operable to produce flue gas having large particulate ash entrained therein, a outlet duct coupled in flow communication with the combustion device so as to receive the flue gas; a screen positioned at least partially in the outlet duct downstream of the combustion device, the screen effective to trap large particulate ash while permitting flow of the flue gas therethrough; and apparatus operable to remove large particulate ash from the screen while at least a portion of the screen remains inside the duct.
 2. The apparatus of claim 1 wherein the screen is in the form of a belt extending across the outlet duct and wrapped around a pair of spaced-apart rollers.
 3. The apparatus of claim 2 further comprising an electric motor arranged to drive one or more of the rollers.
 4. The apparatus of claim 1 wherein a portion of the screen is positioned above or within a hopper positioned outside the outlet duct.
 5. The apparatus of claim 4 wherein the hopper contains a liquid wash solution.
 6. The apparatus of claim 1 wherein the screen comprises a barrier having a plurality of holes formed therein, the holes having a diameter of about 0.1 mm to about 10 mm.
 7. The apparatus of claim 1 further comprising an injector operable to inject alkaline material having a particulate size greater than about 0.1 mm into the outlet duct, upstream of the screen.
 8. A method of removing large particulate ash from a gas stream, comprising: passing a combustion gas stream having large particulate ash entrained therein through a duct, collecting large particulate ash on a screen positioned at least partially in the duct, while permitting flow of the flue gas therethrough; and removing the ash from the screen at a location external to the duct while at least a portion of the screen remains inside the duct.
 9. The method of claim 8 further comprising passing the gas stream to a selective catalyst reduction unit downstream of the screen.
 10. The method of claim 8 wherein large particulate ash is removed from the screen by: moving a portion of the screen outside the duct; and removing large particulate ash from the portion of the screen which is outside the duct; and returning the portion of the screen to the duct.
 11. The method of claim 10 wherein the screen is in the form of a belt extending across the duct and wrapped around a pair of spaced-apart rollers, and the screen is moved by rotating the rollers.
 12. The method of claim 8 further comprising passing a portion of the screen above or within a hopper positioned outside the duct and depositing large particulate ash removed from the screen in the hopper.
 13. The method of claim 12 wherein the hopper contains a liquid wash solution.
 14. The method of claim 8 wherein the screen comprises a barrier having a plurality of holes formed therein, the holes having a diameter of about 1 mm to about 8 mm.
 15. The method of claim 8 further comprising: injecting alkaline material effective to absorb vapor phase contaminants and having a particulate size equal or larger to large particulate ash into the outlet duct upstream of the screen; collecting the alkaline material on the screen; and removing the alkaline material from the screen at a location external to the duct while at least a portion of the screen remains inside the duct. 